isc_socket_create: fcntl/reserved: Too many open files

[john]

Hi,
I'm seeing this quite frequently in syslog from bind:

Dec  7 11:00:00 ext named[26731]: isc_socket_create: fcntl/reserved: Too 
many open files
Dec  7 11:00:00 ext named[26731]: isc_socket_create: fcntl/reserved: Too 
many open files

Googling found someone asked before on here in February and was advised to 
upgrade to 9.3.6, however I'm using 9.5.1-P3 (debian release).

Any ideas how to fix this?

Thanks,

john